Executive Summary : | "The development of clean and sustainable energy technologies has become important to reduce the use of fossil fuels and encounter climate changes. A battery is an electrochemical device that converts chemical energy stored in its active materials into electrical energy through a redox reaction. A battery consists of three main parts cathode, anode and electrolyte. Na-ion batteries are attractive alternative to Li-ion batteries, because of low cost and earth abundance of constituent elements. To achieve high energy density, it is necessary to find suitable anode and cathode which have a larger voltage difference and/or lighter mass. Since cathode materials play a crucial role in the cost, safety, cycle life, and overall energy density of the battery, significant attention has been devoted to exploring suitable candidates. Good cathode material requires (i) high structural and chemical stability, (ii) high voltage and storage capacity, (iii) high electronic and sodium-ion conductivities, and (iv) low cost and safety. Although a wide range of compounds has been explored as plausible Na-ion cathodes, two classes of materials have received an incredible amount of attention, namely, layered oxides and polyanionic frameworks.
Sodium-based layered transition metal oxides (NaxMO2; M = transition metal) have been extensively explored, inspired by the success story of their Li-ion analogues in LIBs. Its crystal structure is built by alternating layers of edge shared MO6 octahedra and Na-ions. According to Delmas et al., Na-ion layered oxides can be categorized into two main groups, O3 and P2 type. The first letter “O” and “P” denotes the types of sodium ion environment (i.e., octahedral and prismatic) and the “2” and “3” implies the number of transition metal layers in a single unit cell. Due to the larger ionic radii difference between sodium and transition metal cations, the cation mixing is avoided in NaMO2 compounds compared to their Li-ion counterparts. This fact enables the preparation of multi-transition metal-based oxides by high-temperature synthesis, which provides optimized SIBs. Oxygen redox reaction (O2-/On-) originates when there is an weak/no overlap between O2p and transition metal ion d-orbitals. The non-bonding like O2p orbital lies higher in energy close to the fermi-level and can donate extra electrons which lead to an extra capacity that solely cannot be achieved by conventional transition metal redox (cationic redox). The first reports were Li-excess cathodes such as Li2RuO3, Li1.2Ni0.2Mn0.6O2, Li2Ru0.5Sn0.5O2 and Li1.3Mn0.4Nb0.3O2. For Sodium-ion batteries the anionic redox reactions are important to elevate the energy density by increasing capacity as well as average operating voltage without compromising the price advantage." |